Fuel cells produce electricity by converting reactants such as hydrogen, hydrocarbons, and oxygen into products such as water and carbon dioxide. In its simplest form, a fuel cell comprises an anode and cathode separated by an electrolyte. The anode and cathode consist of a conductive support, usually carbon black, having a thin layer of a platinum catalyst that is uniformly dispersed over the surface of the support. In a proton-exchange membrane (PEM) fuel cell, the electrolyte is a solid polymeric material capable of conducting protons, e.g., a perfluorosulfonic acid polymer (e.g., Nafion® by DuPont). Examples of these devices are described in U.S. Pat. Nos. 6,030,718, 6,040,007 and 5,945,231 which are incorporated herein by reference.
During operation, a continuous flow of fuel, e.g., hydrogen, is fed to the anode while, simultaneously, a continuous flow of oxidant, e.g., oxygen or air, is supplied to the cathode. In the case of a hydrogen fuel, hydrogen gas is oxidized with the aid of a platinum catalyst at the anode to generate electrons and protons which travel by separate paths to the cathode. The electrons are conducted through an external circuit and the protons are conducted through the electrolyte. At the cathode, oxygen gas combines with the electrons and protons to produce water, again with the aid of a platinum catalyst. The current generated by the electrons flowing through the external circuit can be used for work.
Platinum catalysts are preferred for fuel cells because of their high electrochemical activity. However, platinum is expensive and easily poisoned by the trace amounts of carbon monoxide typically found in hydrogen fuels. Numerous attempts have been made to find less expensive electrocatalysts or reduce the sensitivity of platinum catalysts to carbon monoxide. Several of these attempts have focused on tungsten and molybdenum compounds, and in particular their carbides and oxides. In 1965, AEG-Telefunken discovered that tungsten carbide could potentially replace platinum as an anode catalyst for acid-style fuel cells. These investigators ran a fuel cell for over 30,000 hours with an anode catalyst composed of tungsten carbide. Since that time, five more tungsten compounds have been identified as potential anode and cathode catalysts for low-temperature acid-style fuel cells: WC, W2C, WO3:Pt, NaxWO3, W-POM. For example, U.S. Pat. No. 5,922,488 describes a CO-tolerant anode catalyst which uses a carbon-supported, platinum-dispersed, non-stoichiometric hydrogen tungsten bronze having the formula Pt—HxWO3 wherein x ranges from about 0.05 to about 0.36. U.S. Pat. No. 5,298,343 describes a polycomponent electrocatalyst comprised preferably of platinum or palladium and a chemical component selected from the group consisting of tungstic acid, molybdic acid, ammonium tungstate, ammonium molybdate, sodium tungstate and sodium molybdate. U.S. Pat. No. 5,945,231 contemplates combining tungsten carbide with ruthenium oxide or ruthenium to form a catalysts for a direct liquid-feed fuel cell. Unfortunately, these tungsten and molybdenum-based catalysts have not been shown to exhibit an acceptable level of electrochemical activity for practical fuel cell application without the additional presence of a platinum group metal (platinum, ruthenium, or palladium) co-catalyst.